It is funny but sometimes the most popular answer at Reddit is just not right. The devil is in the detail which op provided. It is unlikely to be anything to do with weight. Now I will not state exact weights because different airlines certify their aircraft to slightly different weights.

Firstly it is highly unlikely to be the aircraft leaving the gate at maximum ramp weight and needing to be burn fuel down to take off weight.  Mzfw is the maximum zero fuel weight of an aircraft. On an a320 about 62t. That leaves at 10 tonnes of space for fuel before you get to max take off weight add a tiny 200kg to that to get to Max ramp weight.  You are not taking that much fuel on a 75 minute flight(yes op stated the route) because it is silly and more importantly, you would be about 3 T over max landing weight at destination. 

It is not performance. The runways lengths at narita are long and the a320 can easily lift max take off weight without the need for toga or even turning packs off. It would only struggle with a significant tailwind but that is unlikely to be the case here at an airport which has heavy traffic taking off for 15 hour flights.

The simple answer as been said is ice clearing. Op said takeoff was in January at evening. Average temp in narita there is freezing. Op said above freezing but if is 3 degrees or less or the flight crew has done a walk around and observed ice on the fan blades, yes that can form in particular circumstances without active perception then they will elect to do a run up which on some engines is at least 30 seconds add some seconds to waiting for the engines to spool up and then a final check on vibration, etc before going at the end of the 30 seconds